217 
(Pl. XLII. and XLV.). The effect of sudden floods, presuma- 
bly those of tributaries but a short distance above Havana, 
appears in February, 1896 (Pl. XLIIL), and in 1899 (Pl. XLV.) 
as a twelve- and two-fold increase respectively, which is re- 
markably abrupt and is followed in both cases by a quick but 
somewhat more gradual return to the previous condition. 
Owing to the complexity alike of the substances included 
in these items of the analysis and of the plankton itself, no 
uniform correlation of these factors can be discovered. ‘l'wo 
different and in a certain sense opposite tendencies can be de- 
tected in the relationship of the movements of the plankton to 
those of the substances under discussion. During the winter 
season and the period of excess of nitrates, plankton pulses are 
attended by increase in the albuminoid ammonia and organic 
nitrogen. This appears in the [linois with the pulses of April, 
1896 (Pl. XLII. ), and December, 1898 (Pl. XLV.). During the 
warmer months, when most of the plankton pulses occur, the 
opposite tendency is seen in the movement of these substances. 
They tend to decrease at times of plankton pulses, as may be 
seen in August and October, 1896 (Pl. XLII), in May, July, 
September, and October, 1897 (Pl. XLIV.), and in June, 1898 
(Pl. XLV.). With the pulses of December, 1895 (Pl. XLIII.), 
and May, 1898 (Pl. XLV.), no marked effect in either direction 
is apparent. 
In Spoon River any seasonal movement of the albuminoid 
ammonia and organic nitrogen is quite thoroughly masked by 
the disturbances due to floods. In 1898 (Pl. XLVII.) these 
substances are a trifle lower in the warmer months than in the 
colder, a condition which may result from the prevalence of 
floods in the latter season. In 1897 (Pl. XLVI.) they increase 
during the warm season and period of low water attending a 
development of the plankton unusual in the water of this 
stream. 
The effect of flood upon the quantity of these substances 
-in the water of this stream is well defined, and seems to throw 
light upon the relation which flood waters bear to the plankton 
